This invention relates in general to turbo machines having a rotating stage, such as a compressor or pump and more particularly to turbo machines subject to operating anomalies known as "surge" and "rotating stall." Most particularly, this invention relates to a method of minimizing or eliminating the occurrence of such surge and stall anomalies, and an apparatus for the practice of such a method.
It is well known that the operating range of modern turbo systems is often limited by the onset of fluid dynamic instabilities. Many types of turbo machines suffer from such instabilities. For purposes of illustration, the example of a compressor is explored fully herein. However, the same principles apply to other turbo machines, such as fans and pumps. Further, the instabilities discussed arise in connection with axial and centrifugal (also known as radial) turbo machines. The invention can be applied to both axial and centrifugal turbo machines.
The instabilities in question are generally categorized as either "surge"or "rotating stall." These instabilities are shown schematically in FIGS. 1a and 1b respectively. Surge is an essentially one dimensional instability characterized by violent oscillations in mass flow through the compressor and pressure rise from the inlet of the compressor to the outlet of the compressor, averaged over an annulus of the compressor. Surge is shown schematically in FIG. 1a, which is a longitudinal cross-section of a compressor unit showing the full length of the compressor.
Rotating stall is shown schematically in FIG. 1b, showing a transverse cross section of a compressor at a point along the length of the compressor. Rotating stall is an essentially two or three dimensional instability in which regions of reduced or reversed mass flow rotate around the compressor annulus. Typically, the frequency of rotating stall is much higher than the frequency of surge.
Each type of instability degrades compressor performance and can lead to catastrophic structural failure of the compressor due to large unsteady aerodynamic loads and increased operating temperatures.
Compressors can be characterized by a non-dimensional performance map or curve showing the relation between the pressure rise coefficient across the length of the compressor and the flow coefficient through the compressor, at a constant rotational speed of the compressor. Such a curve, referred to as the "compressor characteristic," is shown schematically in FIG. 2, at 200. The onset of either rotating stall or surge generally occurs if the operating point is near the peak 202 of the compressor characteristic.
Typically, compressors are operated only in the region where the mass flow coefficient exceeds the mass flow coefficient at the peak 202 by some acceptable margin. This region is sometimes known as the "stable flow" region. The stable flow region is on the negatively sloped, right hand half of the map shown in FIG. 2. In the stable flow region, the compressor exhibits essentially steady, axially symmetric flow. A somewhat simplified explanation of why that region is stable is as follows. If the mass flow rate is reduced, due to some disturbance, from e.g. 0.5, this will cause the compressor to produce a slightly higher pressure rise, since, in this region, a mass flow decrease gives rise to a pressure increase. The increased pressure will tend to increase the mass flow through the compression system, and thus mass flow returns to its steady state value. Similarly, an increase in mass flow will cause a decrease in the pressure difference over the length of the compressor, which will result in less fluid being forced through, thus bringing the mass flow back down to the steady state amount.
Conversely, if the operating point were not in the stable region, for instance if the mass flow coefficient is less than what it would be at the peak 202, the steady, axially symmetric flow is not a stable operating condition. A decrease (or increase) in mass flow causes a decrease (or increase) in the pressure difference from inlet to outlet, which gives rise to a reduction (or increase) in mass flow, followed by another reduction (or increase) in pressure difference. Thus, the steady, axially symmetric flow is not stable, and the compression system settles into a limit cycle, generally in the form of rotating stall or surge. It should be noted that the dynamics of the system also come into play and must be considered to fully describe the phenomenon.
Although, surge and rotating stall generally have different time and length scales, both can be shown to result from the same basic physical mechanism. The mature form of the instability, either rotating stall or surge, has been correlated as a function of a compression system stability parameter (B). See generally, Greitzer, E. M., "The Stability of Pumping Systems--The 1980 Freeman Scholar Lecture," Journal of Fluids Engineering, Vol. 103, pp. 193-242 (1980), which is incorporated herein by reference. However, both forms of instability can occur simultaneously and interact non linearly as they develop.
In recent years, much effort has been directed toward suppressing these instabilities to increase the stable flow range of compression systems. Although fully developed rotating stall and surge are generally large amplitude, highly nonlinear oscillations, each can be viewed as limit cycle oscillation which begins as a small amplitude, essentially linear instability. Thus, rotating stall and surge can be suppressed by modifying the unsteady dynamics of the compression system. Stabilization is achieved by sensing small unsteady flow perturbations and, through a feedback mechanism, generating control actions that damp out the disturbance. Since the disturbances are stabilized at small amplitude, the required control power can be significantly less than the steady state power of the compression system.